1. Field of the Invention
The present invention relates to a lockup damper in a lockup mechanism of a torque convertor, the lockup damper for absorbing and attenuating vibrations transmitted from an input rotary member to an output rotary member.
2. Description of the Related Art
In general, a damper mechanism absorbs and attenuates the vibration transmitted from an input rotary member to an output member while transferring the torque from the input rotary member to the output rotary member. For instance, as an example of the damper mechanism, a damper (hereinafter referred to as a lockup damper) is included in a lockup mechanism disposed in an interior of a torque convertor.
The torque convertor is an apparatus having three types of blade impeller members, an impeller, a turbine and a stator in an interior of the torque convertor for transmitting the torque via a working oil contained in the interior of the torque convertor. The impeller is fixed to a front cover of the torque convertor. Torque is inputted via the front cover from, for instance, an internal combustion engine. The toque is transmitted from the impeller to the turbine by the working oil as the working oil flows from the impeller to the turbine. The working oil then passes through the stator and returns to the impeller. The turbine is connected to a main drive shaft of a transmission and thus the torque is transmitted from the turbine to the transmission.
The lockup mechanism is disposed between the front cover (input rotary member) and the turbine (output rotary member) for selectively mechanically connecting the front cover and the turbine with each other to directly transmit the torque without use of the intervening working oil.
In general, the lockup mechanism includes a piston member that may be pressed against the front cover, a drive member fixed to the piston member, a coil spring supported on the drive member, and a driven member elastically connected with the piston member in the rotational direction via the coil spring. The driven member is fixed to the turbine. The components that form the lockup mechanism constitute the lockup damper for absorbing and attenuating the vibrations that have been received.
When the lockup mechanism operates, the piston member slides along a surface of the front cover or in pressure contact with the surface of the front cover. The torque is transmitted from the front cover to the piston member and further transmitted to the turbine through the coil springs. At this time, the lockup mechanism transfers the torque and at the same time absorbs and attenuates the twist vibrations by the lockup damper. In this case, the coil springs are repeatedly compressed and expand between the drive member and the driven member to thereby absorb and attenuate the twist vibrations.
Recently, in many configurations, in order to reduce an axial dimension of the torque convertor, the lockup damper has been configured with coil springs disposed on an outer circumferential portion of the torque convertor where a relatively large extra space is otherwise unoccupied. However, if the coil springs are arranged around the torque convertor, an angle at which the lockup damper may be twisted is smaller than that of the case where the coil springs are disposed in a radially interior portion or the intermediate portion. In other words, if the coil springs used at a radially inner position are then used at a radially outer position but have the same dimensions, the relative displacement angle possible between the input rotary member and the output rotary member is smaller. As a result, the twist angle of the lockup damper is narrowed so that, in particular, the absorption of twist vibrations in the low RPM region of the engine is reduced.
In order to solve this problem, it is proposed that two coil springs be connected in series with each other through an intermediate member or the like. The circumferential dimensions of the compressible coil springs connected in series with each other is such that it is possible to maintain and possibly increase predetermined angle of relative rotary displacement between the input and output members. Also, if two elastic members having different spring constants are combined in series, it is possible to enhance the twist characteristics of the lockup damper by providing two stage characteristics or the like.
However, if the coil springs are arranged on the outer circumferential portion of the torque convertor as described above, a phenomenon occurs in which a large centrifugal force effects the performance from coil springs. Namely, since a larger centrifugal force than that in the case where the coil springs are arranged in, for example, the radial intermediate portion, may be applied to the coil springs in the operation of the torque convertor, the coil springs are moved radially outwardly, and there is a possibility that the frictional resistance will occur between the coil springs and adjacent members disposed radially outwardly of the coil springs. If the frictional resistance between the coil springs and the other components is increased, it is impossible to sufficiently absorb the twist vibrations.
Also, in the above-described lockup damper, it is necessary to provide a stopper mechanism for limiting the relative rotation to prevent the relative rotary displacement from exceeding a predetermined angle between the drive member nd the driven member. Namely, when the torque that exceeds a certain level is transmitted, the stopper mechanism works and the relative rotation between the drive member and the driven member exceeding the predetermined angle must be prohibited. It may be proposed that, as the stopper mechanism, for example, the coil springs are used so that they are in intimate contact with each other, and the intimately contacted coil springs are used as the stopper mechanism.
However, if the coil springs are thus used as the stopper mechanism, it is necessary to use coil springs which have the sufficient durable strength against the maximum torque load to be transferred, and therefore the range of selection of the coil spring is narrowed. Therefore, the damper characteristics are limited or the cost for the coil springs are increased. In particular, recently, it is desired that the durability of the lockup mechanism be enhanced. Therefore, it is necessary to reduce the load to be imposed to the coil springs. On the other hand, in the case where the coil springs are not used as the stopper mechanism, it is necessary to provide a discrete stopper mechanism. However, if the stopper mechanism is provided discretely, the number of mechanical parts and the number of the working steps to assemble are increased thus increasing the manufacture cost and possibly enlarging the lockup mechanism.
Also, when using two elastic members in series with one another, it is necessary to employ an intermediate member, that may connect the elastic members to each other. In this configuration it is very difficult to stabilize the joint portion between the two elastic member as defined by the intermediate member. For this reason, in repeating expansion and compression of the elastic members, the joint portion is moved radially outwardly, or otherwise in the axial direction of the torque convertor, so that there is a fear that the joint portion will also interfere with or slide along other components to generate an extra frictional resistance. Thus, it is difficult to attain stable damper characteristics in such a lockup damper.